1. Field of the Invention
The present invention relates to an optical glass, a glass preform made of it, an optical product and a process for the production of the optical product. More specifically, it relates to an optical glass which has high-refractivity and low-dispersion optical properties, has a low glass transition point and allows the stable operation of a heat-treating furnace for a long period of time, a glass preform which is made of the above optical glass and suitable for the formation of various optical products, an optical product made of the above optical glass, and a process for the production of the above optical product.
2. Explanation of Related Art
Conventionally, an optical glass having high-refractivity and low-dispersion optical properties contains a large amount of La2O3, Gd2O3, Y2O3, Ta2O5, ZrO2, etc., for attaining a high refractive index and a low dispersion property as is shown, for example, in JP-A-54-90218 and JP-B-54-6042, and glass-network-forming components such as B2O3 and SiO2 are contained in a small amount, so that the above optical glass is extremely highly liable to undergo crystallization. Since a glass that can be stably produced is limited in compositional range, commercially available optical glasses having a high refractive index and a low dispersion have a glass transition point Tg of over 720xc2x0 C. Table 1 shows properties of high-refractivity low-dispersion optical glasses shown in brochures supplied by optical glass manufacturers.
Conventional high-refractivity low-dispersion glasses have a very high temperature for viscous flows as is typically shown by glass transition points Tg, and for example, annealing treatment thereof has required a temperature of 710xc2x0 C. or higher. Generally, most of furnaces for glass annealing are made of a stainless steel plate, and this material has a deformation temperature around 700xc2x0 C. When the annealing is carried out at a temperature over 710xc2x0 C., therefore, there is caused a problem that the above stainless steel plate undergoes deformation, so that it is difficult to operate the furnace for a long period of time.
Further, the production of a lens material by re-heat pressing also requires a very high temperature, which causes a heat-treating furnace to deteriorate sooner and hinders stable production.
Meanwhile, when a glass has a glass transition point Tg of 700xc2x0 C. or lower, stable production has been achieved without causing any special load on the operation of facilities.
Under the circumstances, it is a first object of the present invention to provide an optical glass that has high-refractivity low-dispersion optical properties, has a low glass transition point and allows the stable operation of a heat-treating furnace for a long period of time.
It is a second object of the present invention to provide a glass preform that is made of the above optical glass and is suitable for forming various optical products, and an optical product made of the above optical glass.
It is a third object of the present invention to provide a process for efficiently producing an optical product made of the above optical glass.
For developing optical glasses having the above desirable properties, the present inventors have made diligent studies with regard to effects of compositions of components constituting glasses on optical properties, thermal properties and devitrification resistance. As a result, it has been found that an optical glass having high-refractivity and low-dispersion optical properties and having a glass transition point of 700xc2x0 C. or lower can be obtained by controlling the ratio of the total content of La2O3, Gd2O3, Y2O3 and Yb2O3, and the ratio of the total content of ZrO2, Ta2O5 and Nb2O5, to the total content of SiO2 and B2O3 such that the above ratios come into specific ranges, or by forming a glass having a specific glass composition, and that the first object of the present invention can be accordingly achieved.
Further, it has been also found that the above second object of the present invention can be achieved by a glass preform and an optical product made of the above optical glass each.
It has been further found that a optical product made of the above optical glass can be efficiently produced by employing a specific step, and that the above third object of the present invention can be accordingly achieved.
The present invention has been completed on the basis of the above findings. That is, the present invention provides:
(1) an optical glass having a refractive index nd of at least 1.875, an Abbe""s number xcexdd of at least 39.5 and a glass transition point Tg of 700xc2x0 C. or lower (to be referred to as xe2x80x9coptical glass I of the present inventionxe2x80x9d hereinafter),
(2) an optical glass which is a borosilicate glass comprising at least one selected from La2O3, Gd2O3, Y2O3 or Yb2O3 and at least one selected from ZrO2, Ta2O5 or Nb2O5, wherein the ratio (weight ratio) of the total content of La2O3, Gd2O3, Y2O3 and Yb2O3 to the total content of SiO2 and B2O3 is from 3.2 to 5 and the ratio (weight ratio) of the total content of ZrO2, Ta2O5 and Nb2O5 to the total content of SiO2 and B2O3 is from 1.1 to 1.5, and which has a refractive index nd of at least 1.875 and an Abbe""s number xcexdd of at least 39.5 (to be referred to as xe2x80x9coptical glass II of the present inventionxe2x80x9d hereinafter),
(3) an optical glass which is a borosilicate glass comprising at least one selected from La2O3, Gd2O3, Y2O3 or Yb2O3, at least one selected from ZrO2, Ta2O5 or Nb2O5 and ZnO, wherein the ratio (weight ratio) of the total content of La2O3, Gd2O3, Y2O3 and Yb2O3 to the total content of SiO2 and B2O3 is from 2 to 5, the ratio (weight ratio) of the total content of ZrO2, Ta2O5 and Nb2O5 to the total content of SiO2 and B2O3 is from 0.5 to 3 and the ratio (weight ratio) of ZnO to the total content of SiO2 and B2O3 is at least 0.14, and which has a refractive index nd of at least 1.875 and an Abbe""s number xcexdd of at least 39.5 (to be referred to as xe2x80x9coptical glass III of the present inventionxe2x80x9d hereinafter),
(4) an optical glass having a glass composition comprising, by % by weight, 3 to 10% of SiO2, 7 to 15% of B2O3, 30 to 60% of La2O3, 2 to 8% of ZrO2 and 13 to 19% of Ta2O5, wherein the total content of SiO2 and B2O3 is 14 to 20% and the total content of the above components is at least 95% (to be referred to as xe2x80x9coptical glass IV of the present inventionxe2x80x9d hereinafter),
(5) a glass preform made of any one of the above glasses I to IV,
(6) an optical product made of any one of the above glasses I to IV,
(7) a process for the production of an optical product made of any one of the above optical glasses I to IV, which comprises the steps of melting raw materials for glass and directly press-molding a molten glass, and
(8) a process for the production of an optical product, which comprises the steps of softening a glass preform made of any one of the above optical glasses I to IV under heat and press-molding the glass preform softened under heat.